1. Field of the Invention
The present invention relates to a digital radio communication system for time-division transmission/reception and, more particularly, to a digital radio communication system in which the number of phase-locked loops used for its synchronization is reduced, and a synchronization thereof.
2. Description of the Related Art
Generally speaking, a radio communication system is constructed with a transmitter for modulating information for transmission, and converting the modulated signal into a radio frequency (RF) signal which is then propagated into the air, and a receiver for receiving the RF signal from the air, and converting the radio frequency signal into an intermediate frequency (IF) signal which is then demodulated into the original information. More specifically, the radio communication system's transmitter includes a modulator for modulating the information for transmission, and a up converter for converting the signal modulated by the modulator into a signal suitable for propagation into the air. The receiver consists of a down converter for receiving the RF signal from the air and converting it into the IF signal suitable for processing, and a demodulator for demodulating the IF signal converted by the down converter and thus restoring the original information. Here, a local oscillator for converting up and another oscillator for converting down are connected, respectively, to the up converter and down converter, in order to offer predetermined oscillation frequencies for converting up and down.
The up converting and down converting local oscillators are separately provided and simultaneously connected to the up and down converters in case of an analog radio communication system which performs transmission and reception simultaneously. Meanwhile, for such a digital radio communication system as the global system for mobile communication (GSM), only one local oscillator is offered to be alternately coupled to the up and down converters. I have found that for this reason, a radio communication system for performing time-division transmission and reception needs to have a phase-locked loop able to control the local oscillator so that the local oscillator is in phase alternately with the up converter and the down converter within a predetermined time.
It should be, however, noted that phase-locked loops are often used for removing the frequency deviation of the signal oscillated by a local oscillator at a reference of the frequency produced from a reference frequency generator, are usually independently coupled to respective local oscillators for the purpose of the system's stability by reducing their frequency deviations. As a result, the signals processed at the respective stages of the system are synchronized and stably processed. The number of components required in the system is concomitantly increased however, and thus the cost of the circuit is accordingly raised because the phase locked loops are independently provided to the respective local oscillators. In addition, significantly more control signals are required in order to control the phase locked loops.